1. Field of the Invention
The present invention relates to procedures for forming glass sheets with a view to producing tempered, safety glazings for motor vehicles and/or which have been assembled with the aid of plastic sheets. More specifically, the invention aims at improving forming procedures in which the glass sheets are heated and conveyed in the horizontal position through a furnace by a flat conveyor and are then made convex in a thermally insulated enclosure before being passed into a cooling station.
2. Description of the Related Art
Recent developments in the car industry have led to an ever increasing demand for glazings having complex shapes, with locally, very pronounced curvatures and in particular nondevelopable shapes, derived from spherical, noncylindrical shapes. In parallel, ever higher demands are being made on the optical quality. In the same way, a conformity to the principal curvature with close tolerances is required for all points on the surface of the glazing and not only for those close to its periphery.
Glazings having complex shapes are mainly obtained with the aid of two different technologies.
In the first, which is mainly used for glazings assembled by lamination, such as windscreens, one or two glass sheets are placed cold on a frame, whose peripheral geometry reproduces that which it is wished to give to the glass sheets and then the assembly is heated in a furnace in order to give it a convex shape by gravity. Over the last few years in order to meet the demand for ever more complex shapes, it has been proposed that pressing should take place at the end of the operation using a countermold.
In the second, which is mainly used for tempered or toughened glazings, a clear distinction is made between the stages of reheating the glass sheets, heated in a furnace which they transverse flat, conveyed by a roller bed or an air cushion, and the forming stages, which take place with the aid of tools only acting on the glass when its temperature is at a suitable level.
Thus, in the "tempering" methods, without this term having to exclude the manufacture of laminated glazings, there is a considerable reduction in the number of convex bending tools used and the immobilization time thereof for the production of a given glazing. In view of the very large number of car models, this reduction is a very important advantage, if only for the tool stock management difficulties avoided. In addition, the cycle times are generally much shorter in the case of "tempering" methods.
However, an immediate difficulty of such "tempering" methods using tools of a static nature in the sense that they do not leave the forming cell, is that there is a need for transfers of glass sheets from the conveyor to a first forming tool and in the case of complex shapes from said first tool to a complementary forming tool, which generally cooperates with the first tool and finally a support tool during cooling. All these operations can affect the quality of the glazing, because they lead to risks of marking and poor positioning, so that ultimately there is a poor optical quality and/or nonconformity with the principal curvature.
In addition, numerous so-called tempering processes are a race against time and with the aim of a greater control of the positioning of the tools, it is in fact simpler to place them in the open air outside the furnace. Under these conditions, it is obvious that the glass will cool during its convex bending. However, the tempering stage requires a minimum temperature making it necessary to overheat the glass in the furnace (but then necessarily its optical quality is affected) and/or very rapidly perform the convex bending, which is only possible if the sought shape is of a simple nature. It has admittedly been proposed to preform the glazings before passing them to a convex bending station by pressing between a solid male mold and a female mold, the preforming being obtained by passing the glass at the end of the furnace onto rotary elements such as rollers of the spindle or guide roll type, or onto guide boards. However, these elements are themselves an important source of defects and it is virtually impossible to correctly position the moving glass on such elements. A positioning prior to the entry into the preforming zone is certainly possible, but its effectiveness is relative and once again depends on the complexity of the desired shape, a "simple" shape being much more "tolerant" with respect to the positioning quality. Moreover, the preforming due to rotary elements is cylindrical, but it has been stated hereinbefore that the most complex shapes are spherical, with locally small longitudinal and transverse radii of curvature. Cylindrical preforming only makes it possible to approach one of these radii of curvature.
These cold technologies are opposed by hot technologies in which the forming cell is an integral part of the furnace or is at least thermally insulated so as to maintain therein a temperature substantially identical to the temperature of the glass when it leaves the furnace. It is then possible to extend by a few seconds the time devoted to the forming operation, which on the one hand makes it possible to minimize the temperature on leaving the furnace and on the other hand give more pronounced convex shapings by having adequate relaxing times to avoid any breaking of the glass.
In conventional variants of these technologies, the glass is transferred from the conveyor to a male mold, whose shape it adopts under the action of pressure drop or vacuum (EP 3,391, EP 241, 355) or a rising hot air flow (EP 169,770), or gravity (WO-91/17962). Following this preforming in contact with the male mold, convex bending is completed with the aid of a centrally open, annular frame used in the case of pressing as a female counter-mold or more simply as a support during convex bending due to dropping onto said annular frame (drop forming). In the two former cases, the force acting during the preforming is also used for the transfer between the conveyor and the male mold. In the latter case, a suction pad supports the volume in order to deposit it on a lower, convex mold having a downwardly turned concavity.
It is common to all these processes that a preforming takes place by a solid male mold against which is applied the entire surface of the flat glass sheet, with the possible exception of a small marginal portion. In practice, the same is the case when the transfer between the conveyor and the male mold takes place with the aid of an annular frame, as proposed in European Patent Application Serial Nos. 520,886 and 93.401,165.1, the latter not having been published on the filing date of the present application. Thus, in these processes, the residence of the glass sheet on the annular frame is as short as possible and the different stages can take place in the form described hereinbefore: transfer/preforming on the male mold/convex bending by pressing with the annular frame.